Phosphodiesterases (PDEs) hydrolyze 3′, 5′ cyclic nucleotides to their respective nucleoside 5′ monophosphates. The cyclic nucleotides cAMP and cGMP are synthesized by adenylyl and guanylyl cyclases, respectively, and serve as second messengers in a number of cellular signaling pathways. The duration and strength of the second messenger signal is a function of the rate of synthesis and the rate of hydrolysis of the cyclic nucleotide.
Multiple families of PDEs have been identified. The nomenclature system includes first a number that indicates the PDE family. To date, nine families (PDE1-9) are known which are classified by: (i) primary structure; (ii) substrate preference; (iii) response to different modulators; (iv) sensitivity to specific inhibitors; and (v) modes of regulation [Loughney and Ferguson, in Phosphodiesterase Inhibitors, Schudt, et al (Eds.), Academic Press: New York, N.Y. (1996) pp. 1-19]. The number indicating the family is followed by a capital letter, indicating a distinct gene, and the capital letter followed by a second number, indicating a specific splice variant or a specific transcript that utilizes a unique transcription initiation site.
The amino acid sequences of all mammalian PDEs identified to date include a highly conserved region of approximately 270 amino acids located in the carboxy terminal half of the protein [Charbonneau, et al., Proc. Natl. Acad. Sci. (USA) 83:9308-9312 (1986)]. The conserved domain includes the catalytic site for cAMP and/or cGMP hydrolysis and two putative zinc binding sites as well as family specific determinants [Beavo, Physiol. Rev. 75:725-748 (1995); Francis, et al., J. Biol. Chem. 269:22477-22480 (1994)]. The amino terminal regions of the various PDEs are highly variable and include other family specific determinants such as: (i) calmodulin binding sites (PDE1); (ii) non-catalytic cyclic GMP binding sites (PDE2, PDE5, PDE6); (iii) membrane targeting sites (PDE4); (iv) hydrophobic membrane association sites (PDE3); and (v) phosphorylation sites for either the calmodulin-dependent kinase II (PDE1), the cAMP-dependent kinase (PDE1, PDE3, PDE4), or the cGMP dependent kinase (PDE5) [Beavo, Physiol. Rev. 75:725-748 (1995); Manganiello, et al., Arch. Biochem. Acta 322:1-13 (1995); Conti, et al., Physiol. Rev. 75:723-748 (1995)].
Members of the PDE1 family are activated by calcium-calmodulin. Three genes have been identified; PDE1A and PDE1B preferentially hydrolyze cGMP while PDE1C has been shown to exhibit a high affinity for both cAMP and cGMP. The PDE2 family is characterized as being specifically stimulated by cGMP [Loughney and Ferguson, supra]. Only one gene has been identified, PDE2A, the enzyme product of which is specifically inhibited by erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA). Enzymes in the PDE3 family are specifically inhibited by cGMP. Two genes are known, PDE3A and PDE3B, both having high affinity for both cAMP and cGMP, although the Vmax for cGMP hydrolysis is low enough that cGMP functions as a competitive inhibitor for cAMP hydrolysis. PDE3 enzymes are specifically inhibited by milrinone and enoximone [Loughney and Ferguson, supra]. The PDE4 family effects cAMP hydrolysis and includes four genes, PDE4A, PDE4B, PDE4C, and PDE4D, each having multiple splice variants. Members of this family are specifically inhibited by the anti-depressant drug rolipram. Members of PDE5 family bind cGMP at non-catalytic sites and preferentially hydrolyze cGMP. Only one gene, PDE5A, has been identified. The photoreceptor PDE6 enzymes specifically hydrolyze cGMP [Loughney and Ferguson, supra]. Genes include PDE6A and PDE6B (the protein products of which dimerize and bind two copies of a smaller γ inhibitory subunit to form rod PDE), in addition to PDE6C which associates with three smaller proteins to form cone PDE. The PDE7 family effects cAMP hydrolysis but, in contrast to the PDE4 family, is not inhibited by rolipram [Loughney and Ferguson, supra]. Only one gene, PDE7A, has been identified. The PDE8 family has been shown to hydrolyze both cAMP and cGMP and is insensitive to inhibitors specific for PDEs 1-5. Depending on nomenclature used, PDE8 is also referred to as PDE10, but is distinct from PDE10 described herein. The PDE9 family preferentially hydrolyzes cAMP and is not sensitive to inhibition by rolipram, a PDE4-specific inhibitor, or isobutyl methyl xanthine (IBMX), a non-specific PDE inhibitor. Depending on nomenclature used, PDE9 is also referred to as PDE8, but is distinct from PDE8 mentioned above. To date, two genes have been identified in the PDE9 family.
Specific and non-specific inhibitors of the various PDE protein families have been shown to be effective in treating disorders attributable, in part, to aberrant levels of cAMP or cGMP. For example, the PDE4-specific inhibitor rolipram, mentioned above as an anti-depressant, inhibits lipopolysaccharide-induced expression of TNFα and has been effective in treating multiple sclerosis in an animal model. Other PDE4-specific inhibitors are being investigated for use as anti-inflammatory therapeutics, and efficacy in attenuating the late asthmatic response to allergen challenge has been demonstrated [Harbinson, et al., Eur. Respir. J. 10:1008-1014 (1997)]. inhibitors specific for the PDE3 family have been approved for treatment of congestive heart failure. PDE5 inhibitors are currently being evaluated for treatment of penile erectile dysfunction [Boolell, et al., Int. J. Impotence Res. 8:47-50 (1996)]. Non-specific inhibitors, such as theophylline and pentoxifylline, are currently used in the treatment of respiratory and vascular disorders, respectively.
Given the importance of cAMP and cGMP in intracellular second messenger signaling, there thus exists an ongoing need in the art to identify additional PDE species. Identification of heretofore unknown families of PDEs, and genes and splice variants thereof, will provide additional pharmacological approaches to treating conditions in which cyclic nucleotide pathways are aberrant, as well as conditions in which modulation of intracellular cAMP and/or cGMP levels in certain cell types is desirable. Identification of family-specific and enzyme-specific inhibitors will permit development of therapeutic and prophylactic agents which act on desired cell types expressing PDEs and/or particular metabolic pathways regulated by cyclic nucleotide monophosphate steady-state concentrations.